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COPPER

with about 100 c.c. of water containing a little alcohol. The liquid is
filtered, the filter washed with water acidified with sulphuric acid and con-
taining alcohol (in aU 35 c.c. of alcohol should be used) and the liquid, about
300 c.c. in volume, electrolysed to determine the bismuth.1 ND,00, i.e.,
current per 100 sq. cm. of electrode surface, = o-i amp. ; duration =
about 48 hours; maximum quantity of bismutL which can be deposited
= 0-1 gram.

(6) GRAViMETRiCALLY.2 io grams of the sample are dissolved in 60
c.c. of nitric acid (D 1-3) and the excess of acid expelled by evaporation on
a water-bath. The residue is dissolved in 400 c.c. of water and neutralised,
with continual shaking, by means of a very dilute solution of sodium
hydroxide. As the acidity diminishes, more and more dilute alkali should
be used in order to prevent separation of large clots of copper hydroxide.
A slight excess of the alkali is added so as to produce a faint permanent
turbidity, the liquid being made up to a litre and heated for an hour on
the water-bath, a few drops of the alkali solution being added should the
turbidity tend to disappear. A little further sodium hydroxide solution
is then added to form a just perceptible precipitate and, after 15-20 hours,
the precipitate—containing, besides copper, ah1 the bismuth, iron, etc.,
of the sample—is collected on a filter, washed with cold water and dis-
solved in hot dilute hydrochloric acid. The bismuth is precipitated by
rendering alkaline with ammonia, the excess of which is expelled on the
water-bath (the copper should not precipitate), and the precipitate filtered
off and washed with hot water. The precipitated bismuth hydroxide is
redissolved in hydrochloric acid, the liquid diluted and precipitated with
hydrogen sulphide, the precipitate filtered off and washed with yellow
ammonium sulphide to dissolve any traces of antimony present and then
with water. The bismuth sulphide is then dissolved in nitric acid and
reprecipitated with ammonia, the precipitate filtered off, washed, dissolved
in nitric acid and the solution evaporated in a tared porcelain crucible, the
bismuth oxide being gently heated and weighed : Bi203 X 0-8965 = Bi.

3. Determination of the Arsenic.—5 grams of the sample3 in
fine borings are introduced into the flask used for arsenic distillation (see
Iron, 6) and are gently shaken while 100-125 c.c. of cone, hydrochloric acid
containing in solution 50 grams of ferric chloride free from arsenic 4 are
added through a long-stemmed funnel. The flask is closed with the stopper
carrying the pressure-regulating apparatus, connected with the pipette
dipping into the ammonia solution, and heated gently to dissolve the metal
completely; the flame is then increased and distillation carried on until
the ammoniacal solution becomes faintly acid. At this point the distilla-
tion is suspended, the pipette removed and washed, and the arsenic deter-
mined iodometrically as indicated on p. 179.

1  The lead sulphate which separates, dissolved in nitric acid containing a little am-
monium nitrate and copper nitrate (67 c.c. of nitric acid of D 1-33, 40 c.c. of ammonia
of D 0-923 and2-3 grams of copper nitrate), may be electrolysed to determine the leap
(see 6).

2  Lunge :  Technical Methods of Chemical Analysis (London, 1911), Vol. II, p. 199.

3  With highly arsenical copper, smaller quantities (1-2 grams) are used and io
grams of ferric chloride are dissolved in the hydrochloric acid per gram of metal taken.

* The purity of the reagents is ascertained by a blank test. presence of the copper.